Demand for batteries with higher energy density, lower cost, or improved safety compared to the state of the art has led researchers to investigate metal-air batteries. Metal-air batteries, or metal-oxygen batteries, employ reactions between a metal negative electrode and an oxygen positive electrode. In non-aqueous metal-air batteries, the positive and negative electrodes are separated by an active ion-conducting non-aqueous electrolyte or other ion conducting medium. Typically, oxygen is not stored internally within the battery, but is accessed from the external environment through the positive electrode. As the battery discharges, oxygen diffuses through the electrolyte to the electrode where it reacts electrochemically.
A battery comprises an assembly of one or more electrochemical cells configured to provide output voltage and/or charge capacity. For the purposes of the present invention, the term “battery” will be used to describe electrochemical power generation and storage devices comprising a single cell as well as a plurality of cells. Also, the term “battery,” as used herein, includes both primary and rechargeable batteries, unless otherwise noted.
Examples of metals used as negative electrode materials in metal-air batteries include lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), aluminum (Al), iron (Fe), and zinc (Zn). Because lithium has relatively high electropositivity and low molecular weight, the Li-air battery is a particularly promising technology for high energy density applications.
As an example of a metal-air battery, non-aqueous Li-air batteries are generally governed by the following overall reaction:2Li+O2<-->Li2O2.During the discharging process, lithium oxidizes in the negative electrode, and insoluble Li2O2, Li2O, and/or Li2CO3 deposits in the pores of the positive electrode, and may displace electrolyte within the pores. During the charging process, Li2O2 decomposes, oxygen evolves into the environment and lithium regenerates in the negative electrode. Due to the gain and loss of oxygen during cycling, the mass of a Li-air cell varies inversely with the state of charge. Based on the mass and density of Li2O2, Li-air batteries have maximum theoretical gravimetric and volumetric energy densities of approximately 3,500 Wh/kg and 8,000 Wh/l, respectively (excluding O2), although these numbers are not reached practically. Accordingly, high theoretical capacity is a major driver for the development of Li-air battery technology. For comparison, current commercially available lithium-ion batteries achieve gravimetric and volumetric energy densities of 200 Wh/kg or 500 Wh/l, respectively.
The practically attainable energy density of Li-air batteries is lower than theoretical values due to factors such as the mass of electrochemically inactive components in the battery, such as the electrodes, separator, current collectors, housing, etc. The positive electrode includes the following operations in a Li-air battery: (1) it constitutes a transport system for Li+, oxygen, and electrons to reaction sites; (2) it provides an electrode/electrolyte interfacial area (“active sites” or “electroactive sites”) for electrochemical oxidation and reduction; and (3) it acts as a storage system for discharge products. Many practical limitations in the rate capability and discharge capacity of Li-air batteries arise from limitations of the gas diffusion electrode.
Li-air batteries are commonly fabricated with a flooded gas diffusion electrode, meaning the open pore volume of the electrode is filled with electrolyte. Previous investigations involving flooded gas diffusion electrodes have shown that pore utilization is related inversely to both discharge rate and the thickness of the flooded gas diffusion electrode. This phenomenon can be caused by low oxygen permeability in the electrolyte relative to that of Li+. As discharge rate increases, the distribution of current within the bulk of the electrode becomes increasingly non-uniform. In operating regimes in which oxygen diffusion is rate limiting, the current distribution exhibits decreasing current density with increasing distance from the surface of the electrode in contact with air. The rate of deposition of discharge products in a given region of the electrode may be proportional to the current density in that region. Thus, at higher rates, products deposit faster near the surface of the electrode until a layer is formed that occludes the transport of oxygen, whereupon battery discharge ceases. According to this model, the cessation of discharge occurs sooner at higher rates of discharge, leading to lower overall capacity.
Some challenges that can exist with current Li-air batteries include insufficient rate capability and undesirably low capacity, which are each related to poor utilization of the gas diffusion electrode due the suboptimal transport of reactants and storage of solid discharge products. Li-air cells that achieve high capacity and high rate relative to the state-of-the art are therefore desired. In order to enable practical Li-air batteries for commercial energy storage applications, gas diffusion electrodes must be designed to solve problems such as the foregoing.